before collection (Fig. 5.10). Typical gains achieved by this process are on the order
of a few hundred. This higher gain allows shorter amplifier shaping times and
contributes to better timing resolution.
For synchrotron applications, APDs are often used where the key issue is speed.
Since the high-field drift velocity in Si is about 100 μ ns
À1 , the leading-edge signal
from a 100-μ-thick device will have a rise time on the order of 1 ns. The trailing
lifetime will be determined by the device capacitance, but that is less of an issue for
many experiments. By using very thin devices, time resolutions of ~120 ps have
been achieved [189]. To achieve sufficient stopping power at higher X-ray energies
with such thin devices, individual APDs can be stacked (Fig. 5.10) and/or tilted
[190], or the X-ray photons can first be converted to visible light by high-Z
scintillators such as Pb-doped plastics [191]. As with SDD detectors, arrays of
APDs can be used for higher total count rates—a 10Â10 APD array is under
development for the high-energy photon source (HEPS) in Beijing [192].
5.12 Streak Cameras
The speed record for detectors is unambiguously held by streak cameras or streak
detectors. A streak camera is a time-resolving detector, which works similar to an
old-fashioned TV tube. The key components are a (1) photocathode, (2) a strong
electric field for photoelectron acceleration, (3) sweeping plates, (4) an electron lens,
and (5) a phosphor screen for imaging the photoelectrons. X-rays that are absorbed
in the photocathode release a pulse of photoelectrons, which is both accelerated by a
strong electric field and deflected by a fast voltage ramp on two parallel deflector
plates. The early electrons see a low electric field and are deflected less than later
electrons, which are deflected more. The intensity vs. time information is thus
Fig. 5.10 Left: fundamentals of APD operation. Right: a stack of 16 thin Hamamatsu S5344 APDs
(3 mm diameter, thickness 30 μm) used at PETRA-III, with overall time resolution of about 0.6 ns
(250 ps time resolution for a single element)
5.12 Streak Cameras
121
of a few hundred. This higher gain allows shorter amplifier shaping times and
contributes to better timing resolution.
For synchrotron applications, APDs are often used where the key issue is speed.
Since the high-field drift velocity in Si is about 100 μ ns
À1 , the leading-edge signal
from a 100-μ-thick device will have a rise time on the order of 1 ns. The trailing
lifetime will be determined by the device capacitance, but that is less of an issue for
many experiments. By using very thin devices, time resolutions of ~120 ps have
been achieved [189]. To achieve sufficient stopping power at higher X-ray energies
with such thin devices, individual APDs can be stacked (Fig. 5.10) and/or tilted
[190], or the X-ray photons can first be converted to visible light by high-Z
scintillators such as Pb-doped plastics [191]. As with SDD detectors, arrays of
APDs can be used for higher total count rates—a 10Â10 APD array is under
development for the high-energy photon source (HEPS) in Beijing [192].
5.12 Streak Cameras
The speed record for detectors is unambiguously held by streak cameras or streak
detectors. A streak camera is a time-resolving detector, which works similar to an
old-fashioned TV tube. The key components are a (1) photocathode, (2) a strong
electric field for photoelectron acceleration, (3) sweeping plates, (4) an electron lens,
and (5) a phosphor screen for imaging the photoelectrons. X-rays that are absorbed
in the photocathode release a pulse of photoelectrons, which is both accelerated by a
strong electric field and deflected by a fast voltage ramp on two parallel deflector
plates. The early electrons see a low electric field and are deflected less than later
electrons, which are deflected more. The intensity vs. time information is thus
Fig. 5.10 Left: fundamentals of APD operation. Right: a stack of 16 thin Hamamatsu S5344 APDs
(3 mm diameter, thickness 30 μm) used at PETRA-III, with overall time resolution of about 0.6 ns
(250 ps time resolution for a single element)
5.12 Streak Cameras
121